In a typical casting process, molten metal is poured into a pre-formed mould cavity which defines the shape of the casting. However, as the metal solidifies it shrinks, resulting in shrinkage cavities which in turn result in unacceptable imperfections in the final casting. This is a well known problem in the casting industry and is addressed by the use of feeder sleeves or risers which are integrated into the mould during mould formation. Each feeder sleeve provides an additional (usually enclosed) volume or cavity which is in communication with the mould cavity, so that molten metal also enters into the feeder sleeve. During solidification, molten metal within the feeder sleeve flows back into the mould cavity to compensate for the shrinkage of the casting. It is important that metal in the feeder sleeve cavity remains molten longer than the metal in the mould cavity, so feeder sleeves are made to be highly insulating or more usually exothermic, so that upon contact with the molten metal additional heat is generated to delay solidification.
After solidification and removal of the mould material, unwanted residual metal from within the feeder sleeve cavity remains attached to the casting and must be removed. In order to facilitate removal of the residual metal, the feeder sleeve cavity may be tapered towards its base (i.e. the end of the feeder sleeve which will be closest to the mould cavity) in a design commonly referred to as a neck down sleeve. When a sharp blow is applied to the residual metal it separates at the weakest point which will be near to the mould (the process commonly known as “knock off”). A small footprint on the casting is also desirable to allow the positioning of feeder sleeves in areas of the casting where access may be restricted by adjacent features.
Feeder sleeves may be applied directly onto the surface of the mould cavity, or they may used in conjunction with a breaker core. A breaker core is simply a disc of refractory material (typically a resin bonded sand core or a ceramic core or a core of feeder sleeve material) with a hole in its centre which sits between the mould cavity and the feeder sleeve. The diameter of the hole through the breaker core is designed to be smaller than the diameter of the interior cavity of the feeder sleeve (which need not necessarily be tapered) so that knock off occurs at the breaker core close to the mould.
Moulding sand can be classified into two main categories; chemical bonded (based on either organic or inorganic binders) or clay-bonded. Chemically bonded moulding sand binders are typically self-hardening systems where a binder and a chemical hardener are mixed with the sand and the binder and hardener start to react immediately, but sufficiently slowly enough to allow the sand to be shaped around the pattern plate and then allowed to harden enough for removal and casting. Clay-bonded moulding systems use clay and water as the binder and can be used in the “green” or undried state and are commonly referred to as greensand. Greensand mixtures do not flow readily or move easily under compression forces alone and therefore to compact the greensand around the pattern and give the mould sufficient strength properties, a variety of combinations of jolting, vibrating, squeezing and ramming are applied to produce uniform strength moulds at high productivity.
Moulding practices are well known and are described for examples in chapters 12 and 13 of Foseco Ferrous Foundryman's Handbook (ISBN 075064284 X). A typical process known as the no-bake or cold-setting process is to mix the sand with a liquid resin or silicate binder together with an appropriate catalyst, usually in a continuous mixer. The mixed sand is then compacted around the pattern by a combination of vibration and ramming and then allowed to stand, during which time the catalyst begins to react with the binder resulting in hardening of the sand mixture. When the mould has reached a handleable strength, it is removed from the pattern and continues to harden until the chemical reaction is complete.
When feeder sleeves are employed, they are placed on the pattern plate and the mixed sand applied around them. Typically the mould with the pattern plate and feeder sleeve(s) is part filled with mixed sand which is compacted onto the pattern plate and around the feeder sleeve(s). Further mixed sand is quickly added to fill the mould and the sand compacted, allowed to harden and then removed from the pattern plate. Problems often arise due to poor or insufficient compaction of sand around the base of the feeder sleeve that can lead to poor surface finish and defects in the casting. This is a particular concern when using neck down or tapered sleeves that lead to undercuts between the pattern plate and under the tapered sidewall (neck) where it is difficult to compact the sand consistently and to the required level.
The solution offered in EP-A-1184104 is a two-part feeder sleeve. During the moulding operation, pressure is applied to the top of the sleeve and one element of the sleeve part telescopes into the other. One of the sleeve parts is always in contact with the pattern plate, and the outer upper sleeve element moves towards the pattern plate and compresses the moulding sand underneath it and adjacent to the pattern plate. However, a problem arises from the tabs or flanges which are required to maintain the initial spacing of the two mould (sleeve) parts. During moulding, these small tabs break off (thereby permitting the telescoping action to take place) and simply fall into the moulding sand. Over a period of time, these pieces will build up in the moulding sand. The problem is particularly acute when the pieces are made from exothermic material. Moisture from the sand can potentially react with the exothermic material (e.g. metallic aluminium) creating the potential for small explosive defects.